1 Answer
1

I suspect that the moderator looked at the age of the question and decided that while it is, in all probability a duplicate, the normal process of community votes would be sufficient for the question to get closed in a reasonable time.

Flags should only really be used in cases where the community can't act. In the case of unclosed duplicates it would be when the question is quite old (> 60 days, say) and is therefore unlikely to get the views that will get converted into close votes.